Project Summary: The goals of this application are to understand to what extent replication fitness influences the emergence of HIV variants resistant to non-nucleoside reverse transcriptase inhibitors (NNRTIs), and to delineate the underlying biochemical mechanisms for how drug resistance mutations interact to influence replication fitness. NNRTI-resistant variants of HIV, usually cross-resistant to other NNRTIs, develop rapidly if viral replication is not suppressed. We have shown that HIV replication fitness, as measured in cell culture, is reduced in NNRTI-resistant mutants that are infrequent clinically, even though they confer higher levels of resistance than the most common mutant, K103N. These poorly replicating mutants also reduce RNase H cleavage by reverse transcriptase. Two mutants, G190S and A, also reduce priming from tRNA(Lys,3), raising the question of whether defects in RNA priming also contribute to the reduced fitness of NNRTI- resistant mutants. We also have shown that the nucleoside (nRTI) resistance mutation L74V compensates for the reduced fitness of K103N+L100I, but does not improve its reduced RNase H cleavage rates. Published studies by others have shown that L74V reduces RNA priming. We therefore propose that L74V can compensate for the reduced RNase H activity of poorly replicating, highly NNRTI-resistant variants by improving steps in reverse transcription, such as strand transfer and strand displacement synthesis, that compensate for RNase H cleavage defects. We postulate that selection for highly NNRTI-resistant variants that have improved fitness from compensatory nRTI resistance mutations can lead to early virologic failure. These studies have important implications for the more rational design of nRTI-NNRTI combination regimens. During the next funding period, we plan to: 1. Evaluate the association of nRTI and NNRTI resistance mutations in clinical samples. 2. Determine the effects of nRTI resistance mutations on the replication fitness and drug resistance of NNRTI-resistant mutants. 3. Characterize the biochemical basis for how L74V improves the fitness of NNRTI-resistant variants. Relevance: The proposed studies will evaluate what factors lead to the development of drug resistant strains of HIV, how changes in reverse transcriptase function affect replication of HIV, and whether combinations of drugs can be chosen to influence which drug resistant strains develop. [unreadable] [unreadable] [unreadable]